Dirofilariasis is a parasitic disease of animals and occasionally in humans, which may result from infection by a species of Dirofilaria such as D. immitis, D. repens, D. tenuis, D. ursi, D. subdermata, D. lutrae, D. striata and D. spectans. 
Dirofilaria immitis (heartworm) is a parasitic nematode that commonly infects dogs, foxes, wolves, coyotes, and cats. Heartworms may cause serious vascular damage and may be fatal, especially in highly active animals.
The life cycle of D. immitis is well known (reviewed in McCall et al., Adv. Parasitol. 66:193-285, 2008). In brief, a mosquito may become infected when it draws blood from an infected host (e.g. a dog). In the mosquito, microfilariae (mf) develop to the infective larval stage. When the infected mosquito feeds, it may transmit larvae to a new host (e.g. another dog). In the new host, the larvae continue to mature for eight to ten weeks, after which time they move to the right side of the lungs and the pulmonary artery, where they become adult. Adult worms mate and females produce eggs, which develop in utero into the long thin embryos (microfilariae) that are released into the bloodstream. A mosquito that takes in the circulating mf when it draws blood from the infected host starts the cycle again.
D. immitis may be found wherever its vector, the mosquito, is found. Generally, D. immitis may be found on a world-wide basis, but are very common in areas with mild and warm climates.
Macrocyclic lactones (MLs) are often prescribed as therapeutics or prophylactics in the management of D. immitis in veterinary applications. Example MLs include ivermectin (IVM), milbemycin oxime (MO), moxidectin (MOX) and selamectin (SLM). However, resistance to MLs is common in a variety of parasitic nematodes and appears to be developing in D. immitis. A number of tests have been described for the detection of anthelmintic resistance in nematodes of livestock and horses, including, faecal egg count reduction test, the egg hatch test, microagar larval development test and molecular tests based on benzimidazole resistance (reviewed in Coles et al., Veterinary Parasitology 136:167-185, 2006). Prichard et al. (European patent EP 0979278) describes a P-glycoprotein sequence in Haemonchus contortus which may be useful for the diagnosis of ML resistance in parasitic nematodes. However, there remains a need for methods to detect D. immitis (heartworms) that are resistant to a ML.